1. Field of the Invention
The present invention relates generally to the field of electrical power distribution systems and more particularly to an improved arrangement for coordinating operation of protective devices in a circuit to achieve coordination of devices not heretofore possible.
2. Description of the Related Art
Electrical power distribution systems include various protective devices arranged along the circuit for the appropriate and desirable protection of the power system and its components. Typical protective devices include circuit breakers, power fuses, and current-limiting fuses. Upon the occurrence of an overcurrent or fault condition, it is desirable to de-energize and isolate the smallest possible portion of the system. This is accomplished by the coordination of the operating characteristics of the protective devices so that the downstream devices (farthest from the source) operate before the operation of the protective devices located upstream.
In general and for the lower ranges of overcurrents and faults, a relatively good degree of coordination can be achieved with presently available devices. This coordination can, to some degree, be determined from a composite TCC (time-current characteristic) plot of current versus operating time for all of the devices. Further, the coordination can be verified by actual circuit tests under overcurrent and fault conditions.
Many protective devices include an inverse time-current characteristic which provides faster operating times for higher currents. Some devices additionally include an instantaneous portion applicable to currents above a certain range or value, where instantaneous is defined as operating with a minimum or no intentional time delay after the inception and detection of an overcurrent above the instantaneous level. For example, a circuit interrupter of this type is shown in U.S. Pat. No. 4,571,658. This rapid response is accomplished by sensing the rate of rise of the current (di/dt). One or more of the protective devices in the power system may also desirably include current-limiting characteristics.
To achieve coordination of the various devices including inverse-time, instantaneous, and current-limiting characteristics, circuit theory and application practice compare the let-through I.sup.2 t (amperes-squared-seconds) of a downstream device with the minimum melt or minimum response I.sup.2 t of the upstream device. For example, according to prior art coordination techniques, the total clearing I.sup.2 t let-through of the downstream, load-side current-limiting fuse must be less than the minimum melting (or tripping) I.sup.2 t of the source-side device having current-limiting characteristics. For high current ranges where coordination must still be achieved, since the devices with instantaneous characteristic portions have extremely low minimum tripping I.sup.2 t values, coordination could not be achieved except for certain specific matchups of devices. Thus, unless the I.sup.2 t characteristics were suitable for coordination in this manner, it has not been thought possible to coordinate a device having instantaneous characteristics with a downstream current-limiting fuse. These limitations in coordination interfere with a selection of the most desirable protection; that is, to utilize devices with the instantaneous responses along with devices with current-limiting characteristics to provide the best overall protection against cable faults while protecting downstream devices that could otherwise experience currents above their momentary capabilities.